1. Field of the Invention
The present invention relates to a shock absorber, and more particularly, to a self-pumping type shock absorber.
2. Description of the Prior Art
As a shock absorber of a vehicle such as an automobile there is known a self-pumping type shock absorber as disclosed in, for example, Japanese Patent Laid-open Publication 59-159441, which comprises:
a shock absorbing cylinder-piston assembly having a cylinder bore and a piston, said piston including a disk portion and a rod portion and being slidably engaged in said cylinder bore at said disk portion to define first and second chamber spaces for receiving a working fluid therein, said rod portion extending through said second chamber space out of said cylinder bore;
means for providing a first throttle passage for applying a throttling action to the working fluid flowing from said first chamber space to said second chamber space;
means for providing a second throttle passage for applying a throttling action to the working fluid flowing from said second chamber space to said first chamber space;
a pumping cylinder-piston assembly having a cylinder bore and a piston slidably engaged in said pump cylinder bore to define a pumping chamber for the working fluid, said pumping cylinder-piston assembly being incorporated in said shock absorbing cylinder-piston assembly such that said pump cylinder is provided in said shock absorber piston as a bore coaxially formed therein, while said pump piston is provided as a rod means extending in the shock absorber cylinder bore, so as to thereby obtain a pumping operation synchronized with the shock absorbing operation of said shock absorbing cylinder-piston assembly;
a reservoir for storing the working fluid;
means for providing a relief passage for connecting said first chamber space with said reservoir when said shock absorber cylinder-piston assembly expands beyond a predetermined expansion limit;
an accumulator for accumulating a variable volume of the working fluid, said accumulator having means for applying an elevated pressure to the working fluid accumulated therein; and
means for providing a first connection passage for connecting said pumping chamber with said reservoir, including a check valve for allowing the working fluid to flow only from said reservoir to said pumping chamber;
means for providing a second connection passage for connecting said first chamber space with said pumping chamber, including a check valve for allowing the working fluid to flow only from said pumping chamber to said first chamber space; and
means for providing a third passage for connecting said accumulator with said first chamber space.
When such a self-pumping type shock absorber repeats the expansion and contraction strokes, with concurrent repetition of the intake delivery strokes of the pump, the working fluid is pumped up from said reservoir to said first chamber space, so that the shock absorber gradually expands, and when the shock absorber expands to a predetermined expansion limit value, said relief passage connects said first chamber to said reservoir, whereby the shock absorber does not expand any more. Therefore, when the vehicle height decreases due to a heavy load, etc., the vehicle height is automatically resumed to the standard vehicle height while the vehicle wheel bounds and rebounds. Therefore, the shock absorber can self control the vehicle height at the standard level without any motor driven pump.
In order to improve various performances of the automobile, such as driving comfortableness, steerability, etc., the performance of the suspension is essential, and the suspension performance is much dependent on the damping performance of the shock absorber. Therefore, it is desired that the shock absorber has a damping force variable construction. According to an orthodox technical measure, the damping force of a cylinder-piston type shock absorber would be variably controlled by providing a bypass passage for connecting the working fluid chambers on the opposite sides of the piston and incorporating a variable throttle valve in the bypass passage. When the shock absorber is a self-pumping type shock absorber having a working fluid pumping system such as described above, it will be contemplated to incorporate a variable throttle means in the pumping system to apply a variably controlled throttling action to the flow of working fluid in the pumping system.
However, in the above-mentioned prior art shock absorber, the flow of working fluid in said first passage for connecting the pumping chamber with the reservoir is available only during the suction stroke of the pumping system, i.e. the expansion stroke of the shock absorber; the flow of working fluid in said second passage for connecting said first chamber space with said pumping chamber is available only during the delivery stroke of the pumping system, i.e. the contraction stroke of the shock absorber; and the flow of working fluid in said third passage for connecting said accumulator with said first chamber space is the sum of the pump delivery and the volume corresponding to the reduction of the effective space of the shock absorber cylinder bore due to more further insertion of the piston rod therein during the contraction stroke of the shock absorber, while the flow of working fluid through said third passage during the expansion stroke of the shock absorber is only the volume corresponding to the increase of the effective space of the shock absorber cylinder bore due to less insertion of the piston rod. Therefore, none of those passages provides any substantially uniform flow of working fluid to apply a variably controlled additional damping force during both expansion and contraction strokes of the shock absorber.